BLUF: A good one will help your résumé navigate the often opaque and misaligned fiefdom that is corporate HR.
Long version: If they have significant and current experience with hiring processes (especially in your industry), they'll be able to help you with everything from how to word and structure it to maximize your odds of getting through the automated filters nearly every big company uses these days, to figuring out (based on job postings) what the hiring company's HR department is going to care about. Which is often different from what the person doing the actual hiring will care about, but you need to satisfy both.
They also have visibility into other aspects (salary, titles, mobility) that recruiters often can be helpful with, too, but I've found recruiters sometimes have their incentives somewhat misaligned with those of their clients (e.g., like a real estate agent who makes more money if they focus on turnover over deal size, so the homeowner isn't happy to get $15K less for their house than they could, recruiters make their cut when someone gets hired for anything, at any salary, and a small percentage of a salary that's 10% lower is less of a concern to them than being 10% underpaid is to you), whereas a résumé consultant's job is to do their part to ensure you get the interview.
I run a couple businesses and therefore don't really have a résumé myself, but I've probably referred a total of a half people (friends, relatives, and people moving on from my employ to better things) to a close friend who is a résumé consultant, and they in turn have referred many multiples of that, given how happy they were with the results. Virtually all got interviews with the companies they wanted to. (Of course, a good résumé consultant will also tell you when you're completely on-paper-unqualified to enter that job via traditional means.)
I consider myself a fairly capable person, but seeing the work of a highly talented résumé consultant made me realize that that's just not an area I (or the vast majority of most people) are adept at navigating.
It's sort of like hiring an attorney to handle a legal matter one could technically handle without being an attorney. Sure, if you already have a talent for that kind of thing, and are willing to spend at least hundreds of hours working in that field, you'd probably be able to take care of that one task fairly easily, but anything short of that and it's far better to pay an expert.
A few hundred bucks to put one's résumé into the top few-percent of applications is money very well spent.